The invention relates to silicon dioxide on silicon backplates and condenser microphones employing them.
Miniature condenser microphones can be fabricated by etching single crystal silicon and biased using electrets based on silicon dioxide layers on the silicon. Silicon dioxide has been used for many years in memory devices and shows excellent charge storage properties. However, memory devices store charge at the silicon dioxide--silicon interface and are encapsulated for protection against humidity. Electret microphones store charge at the silicon dioxide--air interface and must be open to the atmosphere.
Silicon dioxide absorbs water at moderate humidity levels. Absorbed water causes surface conduction and loss of charge for electret-biased microphones, which then suffer in performance owing to surface leakage. U.S. Pat. No. 4,908,805, which is hereby incorporated by reference, describes reacting silicon dioxide surfaces with hexamethyl disilazane (HMDS) to form a monomolecular coating of non-polar methyl (CH.sub.3) groups to passivate the surfaces so that they do not absorb water.